igneous
Tumbled dark igneous rock, likely basalt
Basalt; commonly composed of plagioclase feldspar, pyroxene, and olivine
AI-generated identification · may be incorrectFine-grained, dark gray to greenish-gray, dull to subtly vitreous luster, with no visible cleavage; Mohs hardness about 6, specific gravity roughly 2.8–3.0. The rounded shape indicates tumbling or water wear, and the image is insufficient for definitive identification.
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Physical properties
Fine-grained, dark gray to greenish-gray, dull to subtly vitreous luster, with no visible cleavage; Mohs hardness about 6, specific gravity roughly 2.8–3.0. The rounded shape indicates tumbling or water wear, and the image is insufficient for definitive identification.
Formation & geological history
Basalt forms when low-silica, iron- and magnesium-rich lava cools rapidly at or near Earth’s surface, commonly in volcanic regions and oceanic crust. Most specimens are geologically young relative to continental rocks, though basalt occurs from Archean to modern ages.
Uses & applications
Used as crushed aggregate, road base, building stone, railroad ballast, and occasionally decorative landscaping material; attractive polished pieces are sold as inexpensive display stones.
Geological facts
Its dark color and fine texture reflect rapid cooling, which prevents large crystals from developing. Greenish areas may reflect olivine, alteration minerals, or surface staining.
Field identification & locations
Look for a dense, dark, fine-grained texture and occasional tiny pale feldspar crystals; a streak test is usually gray, and it commonly does not fizz with dilute acid. Common worldwide, especially Iceland, Hawaii, India, the Pacific Northwest, and ocean floors.
